The present application is directed to a threaded interbody device for implantation between a pair of adjacent vertebrae in order to provide support to the vertebrae and/or promote fusion between the vertebrae.
In the human spine the pad or disc between vertebrae is sometimes damaged or deteriorates due to age, disease, injury or congenital defect. The vertebrae themselves may also become compressed or otherwise damaged. Because of this, surgery is often utilized to place spacers or interbody devices between the vertebrae which provide proper spacing of the vertebrae and which often are also utilized to promote fusion between the vertebrae. When a device of this type is utilized for purposes of promoting fusion, it is often referred to as a fusion cage or an intervertebral fusion device. When utilized to promote fusion, the interbody devices often are windowed and packed with bone so as to promote growth of the bone between the vertebrae. Sometimes bone is packed between a pair of devices that are placed in close proximity to one another between the vertebrae so as to promote growth of bone and, therefore, fusion between the vertebrae.
Interbody devices are typically either generally rectangular in shape or generally cylindrical in shape. The cylindrical devices have an advantage that they can be threadably received between and into the bones themselves. For this purpose, the vertebrae are typically first spaced and then a drill is utilized to create a partial bore in each vertebrae which allows the interbody device to be received between the vertebrae. Because of the space between the bones, the interbody device usually engages the bone""s only along an upper surface and a lower surface thereof. When the cage is of a cylindrical threaded type, the upper and lower surfaces are curved and essentially designed to engage the portion of the vertebrae whereat bone is unremoved during boring to create an opening for the device.
When interbody devices of this type are used, it is desirable that the device support as much surface of bone as possible to provide strength and reduce the likelihood of subsidence of the device into the bone, especially as part of the bone is spongy by nature. The remainder of the structure mainly functions to support the two surfaces, unless the device is also used as a cage within which to pack bone. Because it is also desirable in such structures to maintain weight and volume as low as possible, in order to make the device more compatible with the body, it is also desirable to make the entire device as small and lightweight as possible, while maintaining strength.
Still further, the cylindrical devices are most often threaded in order to bite into the bone of the vertebrae in order to resist inadvertent removal of the devices from between the vertebrae. Therefore the upper and lower surfaces are threaded for this purpose. In the past flat sided segments have been removed and a tool which saddles over the device has been used that slides along the sides thereof to at least partially complete the threads and that can be rotated to allow the device to be screwed between the vertebrae. Consequently, it is desirable to have a side structure that reduces volume, maintains strength and allows for mating engagement with a tool that can both rotate the device and complete the thread during installation.
Finally, devices of this type that have sectors in the sidewalls missing are essentially taller than wide. This leads to a potential for instability in that the device can inadvertently rotate 90xc2x0 during use and partly collapse. In order to avoid this, an additional structure is needed to prevent inadvertent rotation of the device once it is installed.
An interbody or intervertebral spacer device for placement between a pair of spaced but adjacent vertebrae. The device has upper and lower surfaces that are threaded so as to have a helically wound threadform pattern thereon that is discontinuous between the two surfaces, but mateable with a similar threadform on a tool for completion of the thread for use during insertion of the device.
The device has an elongate body that extends along an axis of rotation. The upper and lower surfaces of the body are convex and the two side surfaces are concave in shape. That is, a cross-section of the upper surfaces and lower surfaces and the side surfaces have edges which are generally semi-circular in shape, except that the upper and lower surface are convex or bowed outwardly, whereas the side surfaces are concave or bowed inwardly. Preferably the radii of generation of each of the side surfaces, the upper surface and the lower surface are approximately the same.
The side surfaces join together the outer edges of the upper and lower surfaces on respective sides of the device. In this manner the device has a profile from the front which approximates a double-headed ax.
The devices are utilized in pairs between two adjacent vertebrae. The devices are preferably joined by a bar. Each of the devices has a recess located on the front thereof within which the bar snugly fits such that the bar resists rotation of each device subsequent to installation. This prevents the devices from inadvertently rotating to a non-supporting configuration during use. A set screw joins the bar to each device.
Therefore, the objects of the present invention are: to provide an interbody spacer or fusion cage device that is threaded on upper and lower surfaces thereof and can be screwed between a pair of vertebrae in order to support the vertebrae and/or to promote fusion between the vertebrae; to provide such a device having concave arcuate side surfaces that join the upper and lower surfaces on opposite elongate sides of the device; to provide such a device wherein the structure provides strength while reducing volume and weight; to provide such a device wherein the device easily mates with an insertion tool having external threads that align with the threads of the device to allow screwing of the device between a pair of vertebrae; to provide such a device that can be either solid or partly hollow in order to allow packing with bone chips or the like; to provide such a device allowing a relatively close spacing of a pair of devices in side by side relationship; to provide such a device that allows a substantial opening between a pair of devices in side by side relationship to facilitate packing with bone chips and subsequent fusion between the vertebrae associated with the devices; to provide such a device utilized in a pair in conjunction with a bar connecting the pair to resist inadvertent rotation of the devices during use; to provide such a device that includes feathering or reduced thread depth near the front or anterior end of the device to provide an even surface for engagement with a harder bony region near the anterior end of the vertebrae in order to reduce the likelihood of subsidence of the device into the vertebrae after installation; and to provide such a device that is relatively easy to construct, inexpensive to produce and especially well suited for the intended usage thereof.
Other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention.
The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof.